1. Field of the Invention
The present invention relates to a digital signal processing method, a digital signal processing apparatus, a digital signal recording method, a digital signal recording apparatus, a recording medium, a digital signal transmission method and a digital signal transmission apparatus.
2. Description of the Related Art
Although methods and apparatuses are variously available for high efficiency coding of an audio signal, conventional examples thereof will be explained below. A conventional method is a transform coding method making up one of the blocking frequency subband coding methods in which an audio signal in the time domain is blocked per unit time, and the signal on a time axis for each block is orthogonal-transformed into a signal in the frequency domain thereby to divide the same into a plurality of frequency bands and to encode the same for each frequency band. Another conventional method is subband coding (SBC: Subband Coding) making up one of the nonblocking frequency subband coding methods in which an audio signal in the time domain is not blocked per unit time but coded by being split into a plurality of frequency bands. A high efficiency coding method is also available which is a combination of the above-mentioned subband coding method and the transform coding method. As an example of the above combined high efficiency coding, after the frequency band is split by the subband coding method, the signal for each band is orthogonally transformed into a frequency-domain signal by the above-mentioned transform coding method, and the signal is coded for each band thus orthogonally transformed.
The subband filter used for the subband coding method includes a quadrature mirror filter (QMF: Quadrature Mirror Filter) or the like, for example, which is described in 1976 R. E. Crochiere "Digital Coding of Speech in Subbands" Bell Syst. Tech. J. Vol. 55, No.8, 1976. Also, ICASSP 83, BOSTON Polyphase Quadrature Filters-A new subband coding technique, Joseph H. Rothweiler, describes an equal bandwidth filter splitting method and an apparatus using such a filter as a polyphase quadrature filter (PQF: Polyphase Quadrature Filter).
One of the above-mentioned orthogonal transform methods is such that, for example, an input audio signal is blocked in a predetermined unit time and transformed for each block from time axis to frequency axis by the fast Fourier transform, the discrete cosine transform (DCT) or the modified DCT transform (MDCT) or the like. The above-mentioned MDCT is described in ICASSP 1987 Subband/Transform Coding Using Filter Bank Designs Based on Time Domain Aliasing Cancellation, J. P. Princen and A. B. Bradley, Univ. of Surrey Royal Melbourne Inst. of Tech.
Further, a band splitting is available in which the human acoustic characteristics are taken into consideration in determining a frequency splitting width in quantizing each frequency component split into frequency subbands. Specifically, the audio signal is sometimes split into a plurality of bands (say, 25 bands) by a band splitting method, generally called a critical band, where the high-frequency bandwidth is widened. At this time, in coding the data for each split band, a predetermined bit allocation is performed for each band, or an adaptive bit allocation is performed for each band for coding. When the MDCT coefficient data obtained by the above-mentioned MDCT process is coded by the above-mentioned bit allocation, for example, the MDCT coefficient data obtained by the MDCT process for each block is coded by the adaptively distributed number of bits.
Further, in coding for each band, what is called the block floating (Block Floating: block floating) process is used for realizing a more efficient coding by normalization and quantization for each band. Specifically, when the MDCT coefficient data obtained by the above-mentioned MDCT process is coded, normalization and quantization are carried out corresponding to the maximum absolute value of the MDCT coefficient for each band. As a result, a more efficient coding is accomplished.
Conventionally, the two methods described below are used in the bit allocation technique described above.
In IEEE Transactions of Acoustics, Speech, and Signal Processing, Vol. ASSP-25, No.4, August 1977, the bit allocation is carried out based on the signal magnitude for each band. On the other hand, ICASSP 1980 The Critical Band Coder--digital encoding of the perceptual requirements of the auditory system M. A. Kransner, MIT, describes a method in which a signal-to-noise ratio required for each band is obtained by utilizing the acoustic masking thereby to carry out a fixed bit allocation.
With the above-mentioned conventional high efficiency coding method and apparatus, the bit allocation amount for each band to be quantized is converted into an integer or the like when calculating it. Therefore, the total number of the bits assigned to all the bands generally fails to coincide with the bit rate specified in the coding format. A bit adjustment operation is thus required for attaining coincidence. A method conceivable for this bit adjusting operation consists in determining the order of priority based on the frequency. In such a case, however, the input signal is not considered at all and therefore adaptive adjusting does not occur. Another conceivable method makes the bit adjustment strictly depending on the input signal. In this case, however, it is necessary to take into consideration again the quantization error for all the signal components and the masking effects again or the like. Thus, the adjusting operation becomes very large in scale.